Bend-capable tubular prosthesis

ABSTRACT

A tubular prosthesis having at least two helical loops or discrete loops, linked by a bridge, includes at least two regular struts connected by a regular inflection point forming a regular gap between the regular struts and at least two stagger struts connected by a stagger inflection point forming a stagger gap wherein the regular gap has a size different from the stagger gap.

PRIORITY

This application is a continuation of U.S. patent application Ser. No.14/881,043, filed Oct. 12, 2015, now U.S. Pat. No. 9,956,098, which is acontinuation of U.S. patent application Ser. No. 13/849,312, filed Mar.22, 2013, now U.S. Pat. No. 9,155,642, which is a continuation of U.S.patent application Ser. No. 12/300,985, filed as a U.S. national stageapplication under 35 USC § 371 of International Application No.PCT/EP2007/004407, filed May 16, 2007, now U.S. Pat. No. 8,403,978,which claims priority to United Kingdom Patent Application No.0609841.2, filed May 17, 2006, each of which is incorporated byreference in its entirety into this application.

TECHNICAL FIELD

This invention relates to tubular medical prostheses that are expandablefrom a radially compact disposition to a radially expanded disposition.In the compact disposition, the prosthesis can by delivered to itsoperational site in the body, typically trans-luminally andpercutaneously, using a delivery system which is a sort of catheter.Exemplary of this class of prosthesis is a nickel titanium shape memoryalloy stent for a bodily lumen which is often but not always an arteriallumen. See for example the disclosure of applicant's earlier WO01/32102.

BACKGROUND

Bodily soft tissue is remarkably flexible and needs generally to beflexible. It is difficult for a metal stent to match bodily tissue forflexibility. WO 01/32102 is concerned with flexibility of the stentduring its journey, from outside the body to the operational site withinthe body, as for example the catheter delivery system advances along atortuous path from the point of entry in the body. However, there isalso a need for metal stents (and other prostheses) that are destined tobe installed at a location in the body where severe bending is to beexpected. If the prosthesis could be made more tolerant of severebending after placement, that would be attractive to doctors and theirpatients.

WO 01/32102 shows what could be termed a “classic” self-expanding stentstructure of zig-zag stenting rings composed of struts interspersed bypoints of inflection and with adjacent stenting rings linked axially byconnector portions. In the compact delivery disposition of the stent(FIG. 3 of WO 01/32102) the struts of the zig-zag rings are more or lessparallel to each other so that each point of inflection represents achange of direction for the material of the stenting ring of more orless 180°. As the stent expands to its deployed configuration (FIG. 4 ofWO 01/32102) the radius of the stenting ring expands by movement of thestruts away from each other so that gaps open up between adjacentstruts, and the adjacent points of inflection move further apart (butnevertheless remain spaced at equal intervals to each other around thecircumference of the stenting ring).

The connectors serve to restrain relative circumferential movement ofthe zig-zag rings relative to each other. Thus, if the points ofinflection of adjacent stenting rings are facing each other in thecompact delivery disposition of the stent, as in WO 01/32102, then sowill be these points of inflection still facing each other in theexpanded disposition of the stent. When an expanded stent is subject, inthe body, to extreme bending, so that the longitudinal axis of the stentis no longer a straight line but a pronounced curve (as in a banana),then the facing points of inflection on the inside of the bend approacheach other. The more extreme the bending, the closer the facing pointsof inflection become until, in the end, these facing “peaks” abut eachother or rub past each other.

Any such abutment or rubbing is undesirable. One way to guard against itis to choose a stent design that can be classified as a “peak-to-valley”design rather than a “peak-to-peak” design as seen in applicant's WO01/32102. The art is replete with suggestions for peak-to-valley designsin which the peaks of any one zig-zag stenting ring do not facecorresponding peaks of the next adjacent stenting ring but, instead, arecircumferentially offset to the peaks of the next adjacent stenting ringby half of the gap between two adjacent peaks of the same ring, in theexpanded disposition of the stent. Then, under extreme bending, anyparticular peak on the inside of the bend can advance into the V-shapedspace between two peaks of the next adjacent stenting ring, without anyabutment or rubbing on any portion of the material of the next adjacentstenting ring.

The present invention is concerned with the above-explained problem. Itseeks to improve the in situ bend capability of stents including thoseseen in WO 01/32102. However, the invention also seeks to achieve thisperformance enhancement without sacrificing other attractive qualitiesof stents such as disclosed in WO 01/32102. For example, simplicity ofmodelling of stress distributions within the stent is attractive in themanagement of fatigue performance of metal stents. Manufacturingsimplicity of course facilitates management of cost which should improveaccess to stents, for those people who need them.

Looking at WO 01/32102, one can quickly see that performance in extremebending could be enhanced by extending the length of the portions thatconnect adjacent zig-zag stenting rings. The longer these “bridges” thenthe more the stent can bend without abutment of peaks on the inside ofthe bend. However, large axial gaps between adjacent stenting rings arenot desirable, for then the tissue of the lumen that is being stented bythe prosthesis is relatively unsupported, at least between adjacentstenting rings.

For the same reason, one seeks as far as possible to distribute themetallic material of the stent as evenly as possible over the length andcircumference of the tubular envelope of the stent so that support forthe stented tissue is as even and uninterrupted as possible. This ofcourse indicates that all gaps between all adjacent struts and points ofinflection of the stent matrix should be as constant as possible.Generally, one does not modulate the strut matrix of a stent, over thelength and circumference of the stent, to suit tissue variations withinthe stenting site. Generally, one does not seek to place a particularpoint on the circumference of the stent in opposition to a particularzone of tissue within the stenting site. For these reasons alone, oneseeks a stent matrix that is everywhere the same over the length andcircumference of the prosthesis.

In principle, the problem addressed in the present invention, and thesolution here disclosed, is useful in all classes of stentingprostheses. That is to say, it works with a stent matrix that exhibitsthe (spiral) turns around the axis of a helical stent, as well as with astent that features a stack of endless stenting rings. It works withballoon-expandable stents and resilient self-expanding stents as well aswith nickel titanium shape memory alloy stents.

SUMMARY

Nevertheless, according to the present invention, it is contemplated todepart from the prior good design practice, and deliberately configurethe stent matrix so that it is locally different, at one point at least,around the circumference of at least every second stenting ring withinthe prosthesis. In short, we envisage within each stenting ring at leastone location (we call it a “stagger zone”) where the gap betweenadjacent points of inflection is smaller (but it could be greater) thanthe otherwise constant (or near constant) “wavelength” around thecircumference of the prosthesis for that particular stenting ring.

It will be appreciated that, for any particular pair of adjacentstenting rings, even just one such abnormal gap (stagger zone) could beenough to set up a circumferential offset between the otherwise facing“peak-to-peak” points of inflection that we see in, for example, FIG. 4of WO 01/32102. In practice, however, a design with only one staggerzone on the circumference might not be enough to deliver the desiredcircumferential offset all around the circumference, especially indesigns that feature a large number of struts, in any one turn aroundthe axis of the prosthesis. Four stagger zones are likely to be enough.Designs with two, three or four such zones are presently preferred, butup to 6 stagger zones per turn are readily conceivable, the more so withstent design with a notably small mesh size in their matrix.

As mentioned above, it is the connectors between adjacent stenting ringsthat prevent adjacent stenting rings from rotating around the long axisof the prosthesis relative to each other and thereby preserve thedesigned intended relationship between facing stenting rings and theirrespective points of inflection. Generally, it will be convenient toincorporate the design variation that creates the abnormal gap betweenadjacent points of inflection with the design of the connector zonebetween two adjacent turns. Thus, in perhaps the simplest case, we canenvisage a connector from which two struts extend into a stenting ringor turn on one side of the connector, and two other struts extend in theother axial direction, into the next adjacent stenting ring or turn. Ifthe angle between the first two struts is “normal” for that stentingring, but the angle between the opposite pair of struts, in the other ofthe two stenting rings, is much smaller than “normal”, then the pointsof inflection circumferentially on either side of the connector, thatwould be facing each other in the classic construction of WO 01/32102,will be circumferentially offset from each other, because of thedifferent angle of the struts each side of the connector. Think of theconnector as the trunk of a human body, with the legs of the bodyslightly open and the two arms of the body. Imagine the angle betweenthe upwardly outstretched arms significantly greater than the anglebetween the legs.

Rather than set out a string of statements of invention, and then repeatthem in a set of claims, we opt for economy of text, refrain fromreciting statements of invention, and present various aspects of theabove explained inventive concept in claims appended to thisdescription.

We have stated that setting up a “peak-to-valley” configuration ofotherwise facing inflection zones is conveniently incorporated in thedesign of portions that connect adjacent stenting rings (or helicalturns). However, one can readily envisage that the necessary offset canbe created in a stagger zone that is circumferentially spaced fromconnectors. There is no imperative that the stagger zone must becoincident with the connector zone.

It will be evident that a stagger zone can be created in a number ofdifferent ways. One can locally manipulate the mechanical properties ofthe material of the stagger zone, not just be local modulation of thedimensions of the inflection points or struts within the stagger zone,relative to the rest of the circumference of the stenting ring, butcould also contemplate manipulation of the mechanical properties of thematerial in the stagger zone by local heat treatment or even chemicaltreatment. It is even possible to envisage adding components to anotherwise “classic” stenting ring structure as in WO 01/32102, such as aflexible “tie” between the two struts that are destined to deliver asmaller than usual gap between adjacent points of inflection. When apainter erects an easel, there is a “piece of string” that runs betweenthe front frame of the easel, and the rear strut of the easel, not farabove the ground, where the gap between the front legs of the easel andthe single rear leg of the easel rest on the ground. A classic stentsuch as shown in WO 01/32102 could be modified by inclusion in it ofjudiciously placed “pieces of string” from biologically compatiblematerial, in order to restrain two adjacent struts of selected stentingrings of the prosthesis from opening to the full extent that occurs forall the other struts of that particular stenting ring. (We mention thispossibility not with any expectation that it will be a preferredconstruction, but in the recognition that competitors stimulated by thepresent disclosure might be obliged to turn to such constructions in thehope of avoiding the inventive concept of the present application). Themotto “keep it simple” is rather powerful and the present inventors areproud of a contribution to the art that is essentially simple but shoulddeliver a powerful enhancement of performance.

At this juncture, it is worth noting that a shape memory alloy stent isgiven its “remembered” configuration by a heat treatment. In the case ofWO 01/32102, that heat treatment is given with the points of inflectionarranged “peak-to-peak”. With shape memory alloy stents embodying thepresent invention, however, the heat treatment is with the inflectionpoints arranged as intended, therefore, not peak to peak, but in thecircumferentially staggered arrangement brought about by the staggerzone(s).

In the context of the present invention, attention is directed to thedisclosure of U.S. Pat. No. 7,223,283 and, in particular, drawings FIGS.4 and 5 and the associated text. We see adjacent zig-zag stenting ringswith points of inflection that are more or less “peak-to-peak” in thecompact configuration of drawing FIG. 4, but not so much peak-to-peak inthe expanded configuration of drawing FIG. 5. However, the struts of anyparticular zig-zag stenting ring are not all the same length. Some arerelatively long, some are relatively short, and some are of intermediatelength. By contrast, it appears that the angle that opens up between anytwo struts that diverge from any particular point of inflection isalways the same, so that any technical effect of circumferentialdisplacement of peak-to-peak points of inflection is accomplished not bylocal variation of an angle between two adjacent struts but, rather, bymodulations of the length of adjacent struts in each of the stentingrings. Putting it another way, each connector portion 24 is theintersection of two straight lines in the form of an “X” shape and thisis not what is envisaged with the present invention.

Thinking about the bend capability of any particular stent design, thismight be limited by peak to peak abutment on the inside of the (banana)bend but could also be restricted by incipient buckling of the stentmatrix at some point on the matrix. Clearly, to obtain benefit from thepresent invention, the stent matrix to which it is to be applied must becapable of bending without buckling, to the extent needed to bring thebenefits of the present invention into play. In general, the sparser thepopulation of connectors, the more capacity the lattice will have tobend without buckling.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, and to show moreclearly how the same may be carried into effect, reference will now bemade, by way of example, to the accompanying drawings, in which:

FIGS. 1 and 2 show, respectively content of FIGS. 6 and 2 ofUS/2003/0055485, being a tubular stent structure opened out flat, seenin plan view, in respectively the radially compact and radially expandeddispositions;

FIG. 3 is a reproduction of FIG. 5 of U.S. Pat. No. 7,223,283, being aportion of a tubular stent structure, opened out flat, and in a radiallyexpanded configuration;

FIGS. 4 and 5 reproduce drawing FIGS. 3 and 4 of applicant's WO01/32102, being respectively side views of a stent structure in theradially compact and radially expanded dispositions;

FIGS. 6 and 7 are corresponding views of a stent structure in accordancewith the present invention; and

FIGS. 8 and 9 are views corresponding to those of FIGS. 4 and 5, orFIGS. 6 and 7, showing another embodiment of the present invention.

DETAILED DESCRIPTION

FIGS. 1 to 5 give only the most cursory impression of the wealth ofstent strut matrix proposals contained within the state of the art.However, they provide enough disclosure to set the advantages of thepresent invention in the context of relevant prior art.

Looking first at FIGS. 1 and 2, we see an example of a stent 10 whichutilises circumferential support structures 12 in the form of zig-zagstenting rings. These are spaced apart along the long axis of the stent.They are made up of struts 12 interspersed by points of inflection 18which US/2003/0055485 designates “apex portions”. These apex portionsare mostly free to take up positions, in the expanded FIG. 2 dispositionof the stent, which are governed only by the stresses transmittedthrough the struts of that particular stenting ring, or transmitted tothat stenting ring by the bodily tissue that presses against it.However, there are also connecting struts 16 each of which joins aselected one of the apex portions of one stenting ring with a selectedapex portion of an adjacent stenting ring. From FIG. 1, we see that theconnecting struts have length direction that is in the circumferentialdirection of the stent tube, so that the opposite ends of eachparticular connecting strut 16 are circumferentially spaced from eachother. In the radially compact disposition of FIG. 1, thecircumferentially extending connecting strut 16 spans an interveningapex portion between its two ends. However, when the stent expands intoits radially expanded configuration represented by FIG. 2, thecircumferential length of the connecting strut 16 is not enough to spanacross an intervening apex portion, because the apex portions arrangedaround the circumference of any particular stenting ring have moved awayfrom each other by an amount greater than the length of the connectingstrut. By selecting a connecting strut length that is approximately halfthe distance between two adjacent apex portions of the same stentingring, in the expanded configuration of the stent, one can achieve a“staggering” of the evenly spaced apex portions of one stenting ring,relative to the equal spacing of the apex portions of the next adjacentstenting ring that “faces” the stenting ring of the other end of theconnecting strut.

In consequence, when the stent in its expanded configuration is subjectto longitudinal compression, or when it is bent sharply (so that itslongitudinal axis is not longer straight but arcuate) the facing apexportions on the inside of the bend, or that approach each other as thestent is longitudinally compressed do not butt up against each otherbut, instead, move into the free gap between two spaced apart apexportions of the other of the two facing stenting rings.

In US/2003/0055485, paragraph 0031, it is stated that the geometry ofthe stent in the radially compact delivery disposition of FIG. 1 is“highly flexible” so that it can tolerate axial compression on theinside of a bend as described above. Looking at FIG. 1 (FIG. 6 of the USpublication) it is not immediately evident how the stated flexibility isprovided.

We turn now to FIG. 3, which corresponds to FIG. 5 of U.S. Pat. No.7,223,283. In this case, one can see that the connector portions betweenadjacent zig-zag stenting rings have a very simple construction. Theyare remarkably short in length but, to the extent that they have alength direction, it is parallel to the longitudinal axis of the stent,rather than in a circumferential direction. Accordingly, there is nocircumferential offset between the apex portions facing each other andconnected by the connector strut 24.

Nevertheless, we see from FIG. 3, that there is a circumferential offsetbetween unconnected apex portions of adjacent stenting rings.

The offset is accomplished by providing a range of different strutlengths in any particular stenting ring. In particular, each connector24 is at the apex of a first pair of relatively long struts and a secondpair of relatively short struts. Each stenting ring features struts ofthree different lengths, namely A) short, B) intermediate length and C)long. And we see from FIG. 3 how the strut length progresses around thecircumference of each stenting ring in a sequence ABCCBA. We can see howthe apex portions at the open end of a bifurcation between two A-strutsboth fall within a single gap between two C-struts of the adjacentstenting ring. This overlap occurs periodically around the circumferenceof the prosthesis, on each occasion midway between two adjacentconnector portions 24.

In the compact disposition of the stent of FIG. 3, apex portions are“head-to-head” or “peak-to-peak” all the way around the circumference ofthe stenting cylinder, not just at those locations where a connectorportion 24 lies between the head-to-head apex portions. In this respect,there is reduced flexibility in the compact delivery disposition of thestent device, comparable with that of the device of FIG. 1 describedabove. Again, it is not immediately evident how the device delivers bentflexibility in the compact delivery disposition.

The present applicant specialises in self-expanding stent devices thatare formed from a tubular workpiece of nickel titanium shape memoryalloy (“NITINOL” trademark). The tube is formed into a stent precursorby forming in it a multiplicity of slits (cut by a laser) that it isconvenient to provide all mutually parallel to each other and to thelong axis of the tubular workpiece. One such construction can be seen inFIG. 4, this corresponding to FIG. 3 of applicant's earlier publicationWO 01/32102, the contents of which are hereby incorporated by reference.Reference to the WO document will reveal how, following laser cutting,portions of the tubular workpiece are removed to leave “holes” in theslitted tube, indicated by reference 60 in FIG. 4. It will be grasped bythe skilled reader that provision of these voids in the slitted tubeendows the tube, in its delivery system, confined by a sheath thatprevents premature self-expansion, greater flexibility for the sheath toadvance along a tortuous delivery path to a site of stenting in thebody.

FIG. 5 shows the expanded disposition of the FIG. 4 stent construction.A pattern of zig-zag stenting rings connected by short connectingportions 62 can be readily recognised and it can also be seen that theapex portions (here in designated “points of inflection”) of adjacentzig-zag stenting rings are facing each other, not only across theconnected 62 but also elsewhere around the circumference of the stentingdevice. Thus, when the expanded stenting device of FIG. 5 is subject tosevere lengthwise compressive stress, or severe bending, there is apossibility for facing points of inflection of axially adjacent zig-zagstenting rings to approach each other closely, or even touch on theinside of the bend. The problem being tackled when the present inventionwas made was how to reduce the likelihood of this adverse eventoccurring.

One embodiment of the present invention, which does succeed in settingthe facing points of inflection of adjacent stenting ringscircumferentially offset from each other, will now be described bereference to drawings FIGS. 6 and 7.

FIG. 6 shows a distribution of slits in a tubular workpiece opened outflat, and the similarity with the slit distribution of FIG. 4 isimmediately apparent. Note that FIG. 4 is a view from the side of theworkpiece, and does not show the workpiece opened out flat whereas FIG.6 shows the entire circumference of the tubular workpiece, laid out flaton the page. We see from FIG. 6 that there are two connecting struts 80connecting any two adjacent stenting rings 82. We see at the left-handend of FIG. 6 the terminal stenting ring 84 which has a longer slitlength and is also evident in FIG. 4. We note that FIG. 4 shows theentire length of the stent whereas FIG. 6 shows only a portion of thelength of the stent. Whereas the connectors 80 are interspersed by voids60 just as in FIG. 4, there are no voids between the increased lengthend stenting ring 84 and its next adjacent normal length stenting ring82′. Whereas there are two connector portions 80 between the normallength stenting rings, there are 14 connectors 86 between the endstenting ring 84 and its neighbour 82′.

Noteworthy in FIG. 6 is the pattern of length of the individual slitscut by the laser. In general, the slits have a single length, but thereare two exceptions. The first exception is that the slit length islonger at the end of the tubular workpiece, to form the end stentingring 84. One such slit is marked in FIG. 6 with reference 88. The secondexception is the length of the slit that terminates at one end of eachconnector portion 80. One such slit, of shorter length then the others,is designated in FIG. 6 with reference 90. We need to look at FIG. 7 toappreciate the consequence of short struts 90.

FIG. 7 reveals two of the many connector portions 80. Each connector 80sits between two adjacent stenting rings at the points of inflection 92of any one stenting ring are all to be found on a notional circularlocus that is transverse to the longitudinal axis of the stent. A gap“t” exists between any particular such circular loci D and E facing eachother at periodical intervals down the length of the stent. Within thesetwo circles, we find a circumferential offset between the spaced apartpoints of inflection 92 in circle E and those of the facing circle D.The offset is found everywhere except at the periodically spacedconnectors (in this example there are four) 80.

Turning our attention to the next adjacent pair of facing circles F andG in FIG. 7, the origin of the circumferential offset can be seen in theopposing relationship of the points of inflection 92A, B, C and D. Thegap between apex 92B and 92C on the stenting ring that includes circle Gis the regular gap between two struts of “normal” length. However,because of the reduced slit length 90, the circumferential gap betweenpoints of inflection 92A and 92D is abnormally small so that both pointsof inflection 92A and 92D “fit” in the normal sized gap between apex 92Band 92C of circle G. In the terminology adopted in the presentspecification, the zone that includes points of inflection 92A, B, C andD is designated a stagger zone. From FIG. 7 it is immediately evidentthat providing such a stagger zone by using an abnormally short slitlength has a disadvantage, namely, that the gap between points ofinflection 92A and 92D is shorter than the regular gaps between otherpoints of inflection spaced around circle F. It hardly need be statedthat optimal use of material within a stent matrix calls for a regularmatrix of struts, with a minimal amount of material in the radiallycompact delivery disposition, and a uniformed distribution of thatmaterial in the expanded configuration (all gaps the same size) so as toachieve a maximum ratio of expanded diameter to radially compressdelivery diameter. Deliberately accepting a plurality of unnecessarilysmall gaps around the circumference of each stenting ring will have anadverse effect on this ratio of diameters and is therefore not somethingof itself desirable to stent designers.

Nevertheless, the present invention is attractive, when taken in thecontext of balancing conflicting constraints on the stent designer.Looking at FIG. 6, one can see the evident simplicity of the stent strutand slit arrangement. The government regulatory authorities imposestringent quality requirements on stent manufacturers. For example,stents must meet stringent metal fatigue requirements. Finite elementanalysis of stent designs is of crucial importance. A design that isinherently simple should lend itself to reliable prediction of itsproperties in service. Being able to predict how a stent will performafter it has been installed in a human body is a significant advantagefor stent manufacturers that compete to provide the stents mostattractive to doctors and medical services.

Reference is now made to FIGS. 8 and 9. These have been annotated withreferences the same as are used in FIGS. 6 and 7, to identifycorresponding features. Noteworthy is that there are four connectorsrather than two, connecting adjacent stenting rings. Clearly, each stentring comprises pairs of struts which are unconnected to any pairs ofstruts of an adjacent stent ring. Further is clear that a number ofadjacent stent rings are spaced from each other. FIG. 8 shows theleft-hand end of the stent whereas FIG. 9 shows the full length.Actually, the FIG. 9 stent is not the same as the FIG. 8 stent becauseFIG. 8 shows longer struts in the two end rings but FIG. 9 does not.

In FIG. 9, it is helpful to consider connectors 80A and 80B. The formerhas the asymmetric shape of all the other connectors of zig-zag rings82. The latter has an X shape because it is part of the transition fromthe stagger zone rings 82 to the end ring 84 that lacks any staggerzone. Shown is that the closest end points of a pair of struts of onestent ring and a pair of struts of another stent ring are connected.

The embodiments illustrated in FIGS. 6 to 9 represent only one of amultitude of ways to bring about an angle between struts that isdifferent from the otherwise regular angle between struts around theremainder of the circumference of any particular stenting ring. Forexample, one could locally modify the material of the stenting ring,either in the points of inflection at one or both ends of the strutsthat are to form the abnormal size gap, or by judicious modification ofthe dimensions of those points of inflection or the two struts runningbetween them. Above-mentioned WO 01/32102 contemplates manual removal ofindividual scrap portions to create voids 60. There are voids 60 in FIG.6. If manual intervention is to be relied upon to create the voids 60 inan embodiment of the present invention, then it would not be beyond thebounds of imagination to intervene locally, and manually, at portions ofthe abluminal surface of the workpiece where the properties of thematerial are to be modified locally in order to deliver a gap ofdifferent size, or angle of different size, between two adjacent strutsof any particular stenting ring. One envisages that the material couldbe modified in its composition, by local application of a substance tocause a chemical reaction, or by local application of a substance tomodify the microstructure of the material at that point, or by localapplication of heat or cooling to give the material at that location adifferent thermal history of that of the material of the remainder ofthe stenting ring.

In this context, we incorporate, by reference to it, the disclosure ofWO2001/076508, from the present applicant, which explains how particularstrut configurations can be created by using a jig to hold the workpiecein a particular desired configuration during the heat treatment which“sets” the “remembered” configuration of the struts in the shape memoryalloy. Thinking along these lines, one could use instead of struts ofdifferent lengths a jig that holds the struts in a configuration such asis shown in FIG. 7, when giving the workpiece its “remembered”configuration, so that it should open up at the stenting site to theremembered disposition even if the length of slit 90 is just the same asthe length of all the other slits in the stenting ring.

When thinking of workpieces of every day household dimensions, such ashow to prevent a door opening too far, one would use a strut between thedoor and the frame that has a set length corresponding to the maximumopening that one wishes to impose on the door. In the same way, onecould envisage some sort of collapsing tie to impose a maximum size onthe gap between points of inflection 92A and 92D, that extends betweenthe respective struts between the slit 90. Of course, stents are verysmall, but by no means as small as the nanometer dimensions that are inthe minds of designers of medical devices, so there seems nojustification for dismissing such tie pieces as impracticable. Thereality is that, as stent designs become ever more sophisticated, so therange of applications for stents becomes ever greater and, with that,the demand for stents to mimic ever more closely the flexible behaviourof the original bodily tissue, especially when called upon, from time totime, to bend tightly along its length. The challenge is to build astent that is strong enough to perform the stenting function which isafter all the reason for its surgical implantation in the body while, atthe same time, rendering the prosthesis as soft and bendy as possible inall other aspects. The present invention makes a valuable contributionto this objective.

Stents need not be made of nickel titanium alloy. Another biologicallycompatible material familiar to stent designers is stainless steel.Great efforts are currently being made to use other materials such asbiologically compatible polymers. All such stents can benefit from thepresent invention regardless how they are formed. The illustratedembodiments are not limiting.

Stents need not display the same strut matrix over their entire length.We envisage embodiments in which only part of the length of theprosthesis is given the high flexibility of the present invention. Thus,there may be some turns of the stent matrix that include stagger zones,to deliver flexibility, and other parts of the length of the stent (e.g.end zones, or a mid-length portion) where high flexibility iscontra-indicated, and so no stagger zones need be provided in theseparts of the stent matrix.

The invention claimed is:
 1. A tubular prosthesis comprising: at leasttwo helical turns or discrete stenting rings, comprising: at least tworegular struts connected by a regular inflection point forming a regulargap between the regular struts, the regular gap having a first size; andat least two stagger struts connected by a stagger inflection pointforming a stagger gap, the stagger gap having a second size shorter thanthe first size; and a bridge linking the at least two helical turns ordiscrete stenting rings, the bridge connected to the stagger inflectionpoint.
 2. The tubular prosthesis of claim 1, wherein the regularinflection point adjacent to the stagger inflection point on a firsthelical turn or stenting ring is offset from a corresponding regularinflection point on an adjacent helical turn or stenting ring.
 3. Thetubular prosthesis of claim 1, wherein the bridge is parallel to alongitudinal axis of the tubular prosthesis.
 4. The tubular prosthesisof claim 3, wherein a regular strut of the at least two regular strutshas a different size than a stagger strut of the at least two staggerstruts.
 5. The tubular prosthesis of claim 4, wherein the regularinflection point adjacent to the stagger inflection point on a firsthelical turn or stenting ring is offset from a corresponding regularinflection point on an adjacent helical turn or stenting ring.
 6. Thetubular prosthesis of claim 5, wherein the tubular prosthesis is astent.
 7. The tubular prosthesis of claim 6, wherein the stent is aself-expanding stent.
 8. The tubular prosthesis of claim 7, wherein thestagger inflection point or stagger strut comprises material differentfrom the regular inflection point or regular strut.
 9. The tubularprosthesis of claim 8, wherein the difference is a difference inchemical composition.
 10. The tubular prosthesis of claim 8, wherein thedifference is a difference in crystalline microstructure.
 11. Thetubular prosthesis of claim 5, wherein the stagger inflection point orstagger strut comprises material different from the regular inflectionpoint or regular strut.
 12. The tubular prosthesis of claim 11, whereinthe difference is a difference in chemical composition.
 13. The tubularprosthesis of claim 11, wherein the difference is a difference incrystalline microstructure.
 14. The tubular prosthesis of claim 3,wherein the regular inflection point adjacent to the stagger inflectionpoint on a first helical turn or stenting ring is offset from acorresponding regular inflection point on an adjacent helical turn orstenting ring.
 15. The tubular prosthesis of claim 14, wherein thetubular prosthesis is a stent.
 16. The tubular prosthesis of claim 15,wherein the stent is a self-expanding stent.
 17. The tubular prosthesisof claim 16, wherein the stagger inflection point or stagger strutcomprises material different from the regular inflection point orregular strut.
 18. The tubular prosthesis of claim 17, wherein thedifference is a difference in chemical composition.
 19. The tubularprosthesis of claim 17, wherein the difference is a difference incrystalline microstructure.